July 21 - The 20th Anniversary of the Teton-Yellowstone F4 Tornado

Precursors to Destruction

July 21, 1987 - The date that will linger in the minds of meteorologists across Wyoming for years to come. The date that refers to the worst tornado to be experienced across Central and Western Wyoming. And, unfortunately, it happened in one of the states most beloved places - Yellowstone National Park. Early day surface conditions demonstrated an area of Low Pressure to the west of Wyoming at 12pm MDT (18z).

Figure 1 - Surface Analysis at 18z on July 21, 1987

This area of Low Pressure continued to move northeast through the afternoon, and caused winds in excess of 100 mph to be found at 200 mb. These high winds provided a lifting mechanism for developing thunderstorms over northwestern Wyoming. Figure 2 shows the location of these winds,which are denoted by the darker shades of pink. The image below demonstrates winds at approximately 6pm (00z).

Figure 2 - 200 mb winds at 00Z July 22, 1987 (6pm on July 21)

The Jackson area began to see thunderstorm conditions at around 2:10pm (2010z), as seen in the official weather observer report below (fig. 3). Note the scratched out areas in the observation around the 2010 observation as conditions changed revealing limited visibilities, high winds, and hail.

Figure 3 - Official Observer Sheet from Jackson, WY

Eighteen minutes later, all these elements combined to produce a tornado over Teton-Yellowstone which touched down at 2:28pm MDT.

The Tornado’s Path and Destruction

An extremely rare event over the area, the tornado touched down in northeast Teton County, and was on the ground for 26 minutes. From its starting point, it traveled north-northeast for 24.4 miles (Fig. 4), and had a path width of 1.6 miles on average. The tornado traveled northeast through Gravel Ridge, Enos Lake, Pacific Creek Valley, the Continental Divide, and the Yellowstone River Valley.

Figure 4 - Path of the Teton-Yellowstone Tornado

During its life, tornado went over different levels of elevation during its lifespan, which included the Continental Divide at 10,000 ft in elevation. The tornado’s parent thunderstorm was 42,000 ft high, and produced 72 seperate microbursts. Figure 5 demonstrates the terrain that the tornado passed over, along with damage, and locations of microbursts relative to the tornado’s path.

Along its path, the tornado uprooted and debarked around 1 million trees in the park - many of which were 12-16 inches in diameter. Note the middle image in Figure 4 shows a shaded depiction of damage caused by the tornado, and also note the most destruction was caused in the lower elevations.

The damage caused by the tornado was evaluated by the father of the Fujita Damage Scale, Dr. T. Theodore Fujita. Upon examination of destruction caused by the torndao, he rated it as an F-4 on the original Fujita Scale which was used until 2007. This rating signifies that the tornado had winds of 207-260 mph with devastating damage.